Indicator Sponge Acid Safety Demonstration SCIENTIFIC SAFETYFAX! Introduction Good housekeeping habits are an important part of laboratory safety. Small drops of acid can burn skin, ruin clothes, and damage books. Check your students’ “housecleaning” skills by using an indicator sponge that instantly changes color when acids are wiped up.
Concepts • Laboratory safety • Acids–bases • pH indicator
Materials Congo red indicator, 1 g Water, distilled or deionized, 100 mL Hydrochloric acid, 0.1 M HCl, 1 mL (or vinegar) Beaker, 250-mL
Sodium bicarbonate, saturated solution, NaHCO3, 10 mL Cellulose sponge, light color
Safety Precautions Congo red indicator is a biological stain and may be a body tissue irritant. Acid solutions are corrosive to eyes, skin and other tissue. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Material Safety Data Sheets for additional safety, handling, and disposal information.
Preparation Make a 1% solution of Congo red indicator by adding 1 g of Congo red to 100 mL of distilled or deionized water. Rinse the sponge (including new sponges) with water to remove residual soap, surfactants, or acids. If the sponge is too large for the beaker, of indictor solution, cut the sponge in half. Place the sponge in the Congo red solution, immersing it com- pletely. Wear rubber gloves to keep from staining hands, periodically squeeze out the liquid. Allow the sponge to soak in the liquid for about 15 minutes. Squeeze out as much liquid as possible and rinse the sponge with fresh water a few times. The indicator sponge is now ready to use. The remaining Congo red solution can be used to make additional indicator sponges.
Procedure Place a drop or two of acid solution (such as 0.1 M HCl or vinegar) on the sponge. Show students the color change from red to blue on the sponge. Treat the blue spots with a small amount of sodium bicarbonate solution to regenerate the red color. This demo can be used early in the school year, when students are sometimes careless about cleaning up spills on the lab bench. If there has been an acid spill on a student bench, the visual impact of the color change on the sponge should get the safety lesson across. Also consider cleaning the benchtop with the indicator sponge after the first experiment using acids.
Disposal Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The sponge and Congo red solution may be saved for future demonstrations. Excess Congo red solution may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.
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Discussion Congo red is a dye, a biological stain, and a pH indicator. When a cellulose sponge is treated with Congo red, the dye becomes permanently bonded to the cellulose fiber but is still functional as an acid–base indicator. The color transition is between pH 3.0 and 5.0. Below a pH of 3.0 (very acidic solutions) the indicator is blue. Above a pH of 5.0, the indicator is red. Therefore, even one single drop of acid solution on a desktop will change the indicator from red to blue immediately. The red color can be regenerated by soaking the sponge in a saturated sodium bicarbonate solution. Many other indicators are also stains or dyes and can be used to make indicator sponges. Clayton yellow can be used to make an indicator sponge for bases. The color transition is between pH 11.0 (yellow) and pH 13.0 (red). Unfortunately, strong bases degrade cellulose sponges and they will not be as reusable as the acid indicator sponges.
Connecting to the National Standards This laboratory activity relates to the following National Science Education Standards (1996): Unifying Concepts and Processes: Grades K–12 Evidence, models, and explanation Content Standards: Grades 5–8 Content Standard B: Physical Science, properties and changes of properties in matter Content Standards: Grades 9–12 Content Standard B: Physical Science, structure of atoms, structure and properties of matter, chemical reactions
Reference Bilash, B. B.; Gross, G. R.; Koob, J. K. A Demo A Day; Flinn Scientific: Batavia, IL, 1995; p 182.
Materials for Indicator Sponge are available from Flinn Scientific, Inc. Catalog No. Description C0120 Congo Red, 10 g H0013 Hydrochloric Acid, 1 Molar, 500 mL S0267 Sodium Bicarbonate Solution, Saturated, 500 mL AP1343 Sponge, Cellulose Consult your Flinn Scientific Catalog/Reference Manual for current prices.
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